1. Field of the Invention
The invention relates to the field of aerial vehicles, and in particular, to micro aerial vehicles with flapping wings.
2. Description of the Related Art
Micro aerial vehicles (MAVs) are small, unmanned aerial vehicles that are typically flown by remote control. MAVs can be, for example and not limitation, small airplanes, helicopters, or ornithopters. Although there is no definite list of qualifications that a vehicle must meet to be considered an MAV, the Defense Advanced Research Projects Agency (DARPA) requires that a particular aerial vehicle-must be smaller than 6 inches in any direction or must not have a gross take off weight (GTOW) greater than 100 grams. DARPA also places limits on, among other things, the range, endurance, operational altitude, maximum speed, maximum payload, and cost of manufacture. Under these tight constraints, the size, weight and power available to on-board avionics and actuators is drastically reduced compared to larger, conventional aerial vehicles.
MAVs are useful in several applications because their small size and maneuverability yields several advantages. For example, MAVs can fly in enclosed or partially enclosed areas, such as in buildings and alleyways. MAVs can also fly through and around obstacles that are too large or too close together to be avoided by conventional aerial vehicles. For at least these reasons, MAVs can perform tasks that other, larger aerial vehicles cannot.
Like larger aircraft, MAVs can carry cameras and other payloads. Unlike conventional aerial vehicles, however, an MAV's small size and maneuverability can make it difficult to detect. For this reason, MAVs are particularly useful to the military, as they can carry out various military operations without being detected. In fact, the U.S. military commonly uses small, mid, and large sized unmanned aerial vehicles (UAV) for search and rescue operations and remote intelligence, surveillance, and reconnaissance (ISR) missions.
A typical MAV mission involves flight through or in close proximity to buildings, tunnels, foliage, rubble, and other hazardous areas. These missions require MAVs to maneuver using sharp dives and climbs along with tight-radius turns. The small size of an MAV generally allows such dynamic flight operations within confined spaces. In addition, in some scenarios, MAVs must also fly for extended periods of time. For these reasons, an MAV's control and power systems must provide capability for both dynamic maneuvers and extended flight times.
Many types of micro aerial vehicles exist, including airplane-like fixed-wing models and helicopter-like rotary-wing models. Each of these types has different advantages and disadvantages. Fixed-wing MAVs can currently achieve higher efficiency and longer flight times, for example, and are therefore well suited to tasks that require extended flying time, higher payloads, and larger ground coverage. Fixed-wing MAVs cannot hover or fly backwards, however, and have a limited ability to fly at slow speeds. Rotary-wing MAVS, on the other hand, can hover, fly at slow-speeds, and move in any direction. Rotary-wing MAVs are generally inefficient, however, and so their maneuverability comes at the cost of shorter flight time and lower payload capacities.
The inefficiencies of presently known MAVs are due, at least in part, to aerodynamics. From an aerodynamic standpoint, MAVs operate in a very sensitive Reynolds number regime. This sensitivity is due in large part to the small size of the wings, rotors, and/or other lifting surfaces on an MAV. The small wings or rotors cause the aerodynamic flow over the lifting surfaces to exhibit strong variances from conventional aerodynamic effects seen over the wings of larger, conventional vehicles. These variances can cause inefficiencies if larger vehicles are simply scaled down to MAV size, or smaller. For this reason, designing MAVs that can efficiently fly in this regime represents a unique and difficult challenge to design engineers. In many applications, for example, it is desirable for an MAV to hover and/or have vertical take off and landing (VTOL) capability. In general, however, conventional VTOL capable vehicles do not efficiently scale down to the small sizes of MAVs. This means that large VTOL capable vehicles cannot simply be reduced to MAV size and maintain high flight efficiencies.
In order to design efficient MAVs, engineers have attempted to model MAVs after small flying animals, such as birds and insects. Birds and insects are notoriously efficient flyers, as their body structure and aerodynamic characteristics are very finely tuned. Birds and insects therefore have unmatched maneuverability, speed, and agility.
In addition, to overcome the aerodynamic difficulties described above, some small birds and insects utilize vortex formation and harnessing to keep themselves aloft, especially when hovering. To reduce the amount of energy needed to flap their wings, many birds and insects also flap at or near their wings' resonant frequency. Flapping at or near the resonant frequency harnesses the vibrational energy of the wings, thereby reducing the amount of energy that the bird or insect must use.
Because certain birds and insects flap their wings at or near resonant frequency, they do not increase or decrease the frequency of the flapping motion in order to vary the amount of thrust produced. Instead, these birds and insects increase the amplitude of the flapping motion to increase thrust, and decrease the amplitude of the flapping motion to decrease thrust. Increasing the amplitude of the flapping motion increases the amount of air displaced by each flap of the wing, thereby increasing the amount of thrust produced by each flap. Decreasing the amplitude of the flapping motion decreases the amount of air displaced by each flap of the wing, thereby decreasing the amount of thrust produced by each flap.
Birds and insects also use elastically stored energy to reduce the amount of energy that must be put into each flap of their wings. When a bird flaps its wings in an upward motion, for example, muscles on the bottom of the bird's body elastically expand and store energy. When the flapping motion reaches it upper peak, these muscles tend to contract like a rubber band, and this elastic energy is used to help pull the wing in a downward flapping motion. Similarly, the downward flapping motion causes muscles on the top of the bird's body to expand and store energy, like a rubber band. When the flapping motion reaches its downward peak, this stored energy is used to help pull the wing in an upward flapping motion. The process is repeated, and the elastically stored energy enables the bird or insect reduce the amount of additional energy that is expended on each flap of its wings.
Engineers have recognized some of the advantages of modeling MAVs after birds and insects. Thus, some biologically inspired MAV designs have been attempted. Some of these designs include the MicroBat, Mentor, and Delfly models. However, all of these models lack appreciable flight time, appreciable payload capacity, the ability to fly in six degrees of freedom (i.e., hovering and VTOL capabilities).
In order to mimic birds and insects, engineers have attempted to design MAVs with flapping wings (ornithopters). The great majority of these designs, however, have been modeled after birds and insects with only two wings. The use of two wings limits the lifting power that can be generated while staying within the MAV sizing parameters. In addition, in prior designs, the wings were not independently controlled. Because the wings were not independently controlled, modifying the flapping amplitude and/or frequency of each individual wing was not possible. These designs also required a tail rudder, elevator, or other flight control mechanisms to perform various flight maneuvers and movement in six degrees of freedom.
Unlike many birds and insects, current designs do not utilize resonance to reduce the amount of energy needed to flap the wings. When an increase or decrease in thrust is needed, these designs simply flap the wings faster or slower, thereby increasing or decreasing the frequency of the flapping motion. This can be an inefficient way to produce and/or modify thrust. For these reasons, known MAVs with flapping wings are inefficient, have low lifting power, and are overly complex. Moreover, the inefficiencies and low lifting power associated with these MAVs require that they either have small power supplies that exhaust quickly, or be tethered to an external power supply.
Accordingly, while engineers have modeled MAVs after some birds and insects, there are many designs that have not yet been attempted. Some of these designs can potentially offer the benefits of both fixed-wing and rotary-wing aerial vehicles, without many of the drawbacks. Specifically, some of these designs can offer an efficient MAV with relatively high payload capacity, the ability to hover, the ability to glide or fly like an airplane, VTOL capability, and maneuverability in six degrees of freedom.
What is needed, therefore, is a more efficient MAV with improved payload capacity that has the capabilities of both fixed-wing and rotary-wing aerial vehicles. It is to such a system and design that embodiments of the present invention are primarily directed.